Wednesday, March 9, 2011

One Valve CW transmitter

This transmitter was first constructed in 1987 and provided the author with his first 'real' rig, capable of distances of more than about 100 metres. It performed better than expected, with 250km contacts being commonplace and 2500km being occasionally possible.

It consists of a 6GV8 valve, common in the many valve TVs that were rusting in rubbish tips at the time. Unlike some slightly simpler designs, it is a two stage circuit, the triode section being used for the oscillator and the pentode as the power amplifer. With a high tension of about 200 volts power output of about 3 watts could be obtained on 3.5 MHz.

It will work with the cheap 3.58 MHz crystals, but you won't get many contacts up there – better to invest in a lower frequency crystal – eg 3.530 MHz or so. A VXO is not provided – these do not usually provide much shift with 3.5 MHz crystals. However if you wish to add one, wire an inductor (approx 6.8uH) in series with the crystal and add a variable capacitor from point 'A' to earth.

A circuit for a power supply is not included. The one used for this project is based on a transformer from an old valve radio. Use a bridge rectifier and electrolytic capacitor (rated at 350v or so) for smoothing. However as the current drawn is low, even a single 1000v diode will work as the rectifier in this circuit.

Like nearly all valve circuits, this transmitter uses lethal voltages. It should be fully enclosed so that no high voltage points can be touched by the operator. A metal chassis, such as a cake tin is suitable.

To test, insert a crystal, wire a small light bulb (eg dial light) across the antenna terminals and press the key. Adjusting the Tune and Load controls should cause the light to glow quite brightly. Plate current should be around 30-50mA. For the transmit/receiving switching, either have the receiver connected to a separate antenna or use a switch or relay to switch the antenna over.